A general three-phase common container-type circuit breaker can be represented by a cylindrical three-phase common tank-type circuit breaker that is employed for a transmission system of higher than 70 kV as shown in FIG. 5, namely in FIG. 1 of Japanese Patent Laid-Open No. 52-27562 distributed on Mar. 1, 1977 and entitled "Gas circuit breaker".
In this circuit breaker, the breaker units of three phases are arranged in the axial direction thereof in a cylindrical container 25 filled with an SF.sub.6 gas, branch tubes 40, 41 are protruded in the radial direction at both ends in the axial direction of the cylindrical container 25, and lead conductors 27, 28 extending in the axial direction of the branch tubes are connected to insulating spacers 26a, 26b that hermetically seal the branch tubes, thereby to form a generally U-shaped current path 27-81-28.
Further, a similar example of three-phase common container-type circuit breaker shown in FIG. 5 can be represented by a cubic type pole switch 2 shown in FIG. 1. The pole switch 2 is constituted at an upper portion of a distribution pole 1 as shown in FIG. 1, and in which distribution lines 29 and 30 are connected to lead conductors 27 and 28 that are lead out nearly horizontally from both ends thereof. Details of the breaker units are constituted in a metal casing filled with the SF.sub.6 gas as shown in FIG. 2 which is a vertical section view of FIG. 1 and as shown in FIG. 3 which is a section view along the line XII--XII in FIG. 2. An insulating cylinder 6 is coupled between a bushing 23a and a bushing 23b mounted on the metal casing 25, and is also filled with the SF.sub.6 gas. The insulating cylinder 6 contains a fixed contact 3 and a moving contact 4 that are electrically connected to the bushings 23a and 23b, respectively, and the moving contact is opened or closed by an operation unit 7 in the metal casing 25 via a drive lever 5. In the insulating cylinder 6 are formed a heating chamber 8 in the vicinity of the fixed contact 3 and a downstream chamber 10 on the side of the moving contact 4. A zero-phase sequence current transformer ZCT for ground protection is arranged to surround three phases along the periphery of the bushing 23a in the metal casing 25, and another current transformer CT for over-current protection is arranged to surround the two-phase bushing 23b, thereby to constitute a sequence that is shown in FIG. 4. The breaker units of three phases are arranged in line in the horizontal direction in the insulating cylinder 6 of a unitary structure as shown in FIG. 3, and are operated by the operation unit 7.
In recent years, it has been studied to replace such a switch having a load current breaking ability only by a pole circuit breaker that has accident current breaking ability. This is because, by imparting accident current breaking ability, the section of power failure can be minimized in case of an accident and the accident can be restored very fast, contributing to strikingly improve reliability in supplying the electric power.
Under the above-mentioned circumstances, it has been studied to upgrade the pole switch shown in FIG. 2 to the pole circuit breaker that has accident current breaking ability, and to constitute the pole circuit breaker using the circuit breaker that is constituted as shown in FIG. 5.
When the conventional cubic type pole switch shown in FIGS. 1 to 3 is to be used as a pole circuit breaker having accident current breaking ability, the zero-phase sequence current transformer ZCT must be arranged at an end of the breaker unit as shown in FIG. 4. According to the conventional constitution in which the lead conductors 27 of three phases arranged horizontally and parallel in line are surrounded as shown in FIG. 2, however, the constitution is in no way desirable for the zero-phase sequence current transformer ZCT that detects ground current based on minute imbalance among the three-phase current, and the precision of detection must be further increased.
When the circuit breaker shown in FIG. 5 is used as a pole circuit breaker, furthermore, it can be contrived to arrange the zero-phase sequence current transformer ZCT in the vicinity of, for example, the insulating spacer 26a. However, the lead conductors 27 positioned at this portion have dissimilar lengths between two phases as shown. Furthermore, a length between support conductors 31 which support the fixed contact of the breaker unit and the lead conductors 27, and the zero-phase sequence current transformer ZCT is different from each other in each phase. Therefore, the constitution is not desirable for the zero-phase sequence current transformer ZCT that detects ground current based upon minute imbalance among the three-phase currents. Furthermore, it is contrived that distribution lines 29 and 30 are led out in a shape of L-type from the U-shaped current path 27-81-28 shown in FIG. 5. However, such an arrangement has same drawback explained in FIG. 5, since the distribution lines 29 and 30 of three phases are not arranged in straight lines but in the shape of the L-type. Accordingly, it is difficult to use the circuit breaker of FIG. 5 as the pole circuit breaker.